Abstract

A dual-channel fiber-coupled laser heterodyne system operating at a 1.55-µm wavelength is used to investigate phase fluctuations induced on a laser beam by propagation through turbulent air. Two receivers are used to characterize spatial and temporal variations produced by a turbulent layer of air in the laboratory. The system is also used for measurements through extended turbulence along an 80-m outdoor atmospheric path. Phase structure functions, power spectral densities, and cross correlations are presented.

Temporal cross correlation of the phase shifts at two points on the wave front for five different vertical separations. The curve for the smallest separation, r = 9 mm, has the highest value at delay time zero, and the others decrease in order down to the largest separation, r = 41 mm.

Values of the normalized cross correlation as a function of separation distance r in the vertical direction. The squares show the values at zero delay and the circles show the peak in the cross-correlation curves. The differences between the two give an indication of the relative importance of evolution and translation of the airflow.

Phase structure function for horizontal separations over the 80-m atmospheric path. The squares are measured values, the circles are values inferred from measurements of the intensity fluctuations, and the solid line is a fit of a 5/3 power law to the measured values.

Four phase difference spectra. The upper trace is for 450-mm probe separation, the second trace is for 30 mm, the third for 6 mm, and the lower trace is a background reading with no atmospheric fluctuations. The solid curves are fits of Eq. (7). PSD, power spectral density.